Seams as in vertical, or between layers or both?
for vertical, cover one half (half prone to settle low, & prop up first) with plastic wrap so the water cannot adhere to one surface, but does rest there nice. Your seam may unseat on the plastic wrap half, but cracking chance is 50% reduced.
For purely leaky çoncerns, seal the land being covered by painting with "water" or compatible paint sealer and letting it set up, then doing an actual pour. Caulk, then color paint, then a water pour is another option if the open gaps and cells are big ones.
If you use a two part mix, it may work to wait until the water thickens slightly to pour. (I've done this with fiberglass pouring, but your timing has to be real good, and pour a fast but smooth blob that can still flow flat, to avoid bubbles in it or even forming stress and stretch streaks .)
